Structural relaxation kinetics of antimony borate glasses with covalent bonding character

Abstract

The structural relaxation kinetics in the glass transition for xSb2O3⋅(100−x)B2O3 glasses with covalent bonding character has been examined from viscosity and heat capacity measurements. These binary glasses have low glass transition temperatures, Tg=250–290 °C, but show a high thermal resistance against crystallization. The degree of fragility m estimated from the activation energy for viscous flow (Eη=290–531 kJ mol−1) is m=29–51. The activation energy for enthalpy relaxation, ΔH=297–602 kJ mol−1, is evaluated from the cooling rate dependence of the limiting fictive temperature. The ΔH values are very close to the Eη values, meaning that the decoupling between enthalpy relaxation and viscous flow is small. The values of Kovacs–Aklonis–Huchinson–Ramos (KAHR) parameter θ estimated from ΔH/RTg2 are 0.13–0.25, where R is the gas constant. The glasses with 30–60 mol % Sb2O3 have very similar Eη, m, ΔH, and θ values. It has been demonstrated that the structural relaxation kinetics of the binary antimony borate glasses is affected by the boron coordination numbers (i.e., BO3 and BO4) and the covalent bonding character of Sb–O bonds, and consequently these glasses are regarded as a highly strong glass-forming system in the fragile/strong classification concept.